1. Field of the Invention
The present invention relates to a field emission-type electron source for emitting electron beams by means of electrical field emission and a manufacturing method thereof, and to a display using the field emission-type electron source.
2. Description of the Prior Art
Conventionally, as a field emission-type electron source, there has been well-known a so-called Spindt-type electrode which is disclosed, for example, in the U.S. Pat. No. 3,665,241 and so on. The Spindt-type electrode is provided with a substrate on which many small (fine) emitter chips of triangular pyramid shape are disposed, emitting holes for exposing apexes of the emitter chips to the outside, and gate layers disposed in such a manner so as to be insulated to the emitter chips. Thus, the Spindt-type electrode can emit electron beams from the apexes of the emitter chips to the outside through the emitting holes by applying high voltage between the emitter chips and the gate layers under a vacuum atmosphere in such a manner that the emitter chips become negative electrodes against the gate layers.
However, in the Spindt-type electrode, there exists such a problem that when it is applied, for example, to a flat light emitter or a display, it is difficult to enlarge its area (electron emitting area), because manufacturing process of the electrode is complicated and further it is difficult to produce many emitter chips or triangular shape with higher efficiency. Meanwhile, in the Spindt-type electrode, the electrical field converges to the apexes of the emitter chips. Therefore, when the degree of vacuum around the apexes of the emitter chips is lower so that residual gas exists thereabout, the residual gas is ionized by the emitted electrons to become positive ions so that the positive ions collide to the apexes of the emitter chips. Accordingly, the apexes of the emitter chips suffer damages (for example, damages due to ion impacts) so that there may be caused such a problem that current density or efficiency of the emitted electrons become unstable, or lives of the emitter chips are shortened. Therefore, the Spindt-type electrode must be used under a higher vacuum atmosphere (about 10.sup.-5 Pa to about 10.sup.-6 Pa) in order to prevent occurrence of the above-mentioned problem. Accordingly, there may be caused such a disadvantage that the manufacturing cost of the electrode is raised and further the method of treatment of the electrode is troublesome.
In order to improve the above-mentioned disadvantage, a field emission-type electron source of MIM (Metal Insulator Metal) style or MOS (Metal Oxide Semiconductor) style has been proposed. The former is a flat field emission-type electron source having a laminated structure of (metal)-(insulator film)-(metal), while the latter is a flat field emission-type electron source having a laminated structure of (metal)-(oxide film)-(semiconductor). In order to raise electron-emitting efficiency of the above-mentioned type of field emission-type electron source (namely, in order to emit more electrons), it is necessary to reduce the thickness of the insulator film or oxide film. However, if the thickness of the insulator film or oxide film becomes thinner to excess, it is feared that dielectric breakdown is caused when voltage is applied between the upper and lower electrodes of the laminated structure. Therefore, there may be a certain restriction on reducing the thickness of the insulator film or oxide film. Thus, there exists such a disadvantage that its electron emitting efficiency (electron extracting efficiency) can not be raised so much, because the above-mentioned dielectric breakdown must be prevented.
Moreover, in recent years, as disclosed in the Japanese Laid-Open Patent Publication No. 8-250766, there has been proposed another field emission-type electron source (semiconductor element for emitting cold electrons), in which a porous semiconductor layer (porous silicon layer) is formed by performing anodic oxidation to one surface of a monocrystalline semiconductor substrate such as a silicon substrate in order to raise its electron emitting efficiency, a thin metal film being formed on the porous semiconductor layer. Hereupon, the electron source can emit electrons when voltage is applied between the semiconductor substrate and the thin metal film.
However, in the field emission-type electron source disclosed in the Japanese Laid-Open Patent Publication No. 8-250766, there exists such a disadvantage that it is difficult to enlarge its area and to lower its manufacturing cost, because the material of the substrate must be restricted to semiconductor. Meanwhile, in the field emission-type electron source, because a so-called popping phenomenon is easily caused when electrons are emitted, the amount of emitted electrons tends to become unsteady. Accordingly, when the electron source is applied to a flat light emitter or a display, there may be caused such a disadvantage that light emission becomes unsteady.
Thus, in the Japanese Patent Applications of No. 10-272340, No. 10-272342 and No. 10-271876 etc., the inventors of the present application have proposed another field emission-type electrode source including a strong field drift layer through which electrons injected from an electrically conductive substrate can drift, the layer being disposed between the electrically conductive substrate and a thin metal film, and the layer being formed by oxidizing a porous polycrystalline silicon layer by means of the Rapid Thermal Oxidation method (RTO method).
Hereupon, the porous polycrystalline silicon layer is formed by making a polycrystalline silicon layer on an electrically conductive substrate porous by means of an anodic oxidation treatment. Meanwhile, the oxidation of the porous polycrystalline silicon layer by the RTO method is performed in a dry oxygen atmosphere using a lamp annealing apparatus. In this case, temperature of the oxidation may be 800-900.degree. C., and time of the oxidation may be 30-120 minutes (Japanese Patent Application No. 10-271876). Further, the thin metal film is formed using a thin gold film of about 10 nm thickness, because electrons (the electrons are considered as hot electrons), which have reached the surface of the strong field drift layer, must be emitted to the vacuum atmosphere in such a manner that they have passed through the thin metal film without being dispersed therein. In the field emission-type electron source, electrons can be stably emitted, because its electron emitting property has a smaller dependency to the degree of vacuum, and further a popping phenomenon is not caused when the electrons are emitted. Further, in addition to a semiconductor substrate such as a monocrystalline silicon substrate, there may be used a substrate in which an electrically conductive film (for example, ITO film) is provided on a surface of a glass substrate or the like, as the above-mentioned electrically conductive substrate. Therefore, in the field emission-type electron source, its area may be enlarged and its manufacturing cost may be lowered, in comparison with the conventional electron source utilizing the porous semiconductor layer produced by making the semiconductor substrate porous, or the Spindt-type electrode. Hereupon, when a display is produced using this type of field emission-type electron source, the thin metal film must be patterned to a predetermined shape.
However, in the field emission-type electron source disclosed in the Japanese Patent Application of No. 10-272340, No. 10-272342 or No. 10-271876, the temperature of the oxidation by the RTO method can not be raised over the heat resistant temperature of the electrically conductive substrate. Therefore, there exists such a disadvantage that materials of the substrate or the electrically conductive film are restricted so that enlarging the diameter (area) of the substrate is also restricted.
Meanwhile, in the field emission-type electron source disclosed in the Japanese Patent Application of No. 10-272340, No. 10-272342 or No. 10-271876, the porous polycrystalline silicon layer, which has been formed by oxidizing the polycrystalline silicon layer by means of the anodic oxidation treatment, is oxidized by means of the RTO method. Hereupon, as an electrolyte solution used in the anodic oxidation treatment, there may be used a solution in which hydrogen fluoride aqueous solution and ethanol are mixed together in the ratio of 1:1.
Hereupon, in the porous semiconductor layer (porous polycrystalline silicon layer or porous monocrystalline silicon layer) formed by the anodic oxidation treatment, silicon atoms are terminated by hydrogen atoms. Therefore, as disclosed in the Japanese Patent Application of No. 10-272340, No. 10-272342 or No. 10-271876, if an oxidized film is grown by the RTO method in the porous semiconductor layer formed by the anodic oxidation treatment, hydrogen atoms may remain in the oxidized film, or Si--OH bonds may be produced. In consequence, there exists such a disadvantage that it is difficult to form an minute oxidized film having SiO.sub.2 structure so that its breakdown voltage is lowered. Further, because the strong field drift layer contains relatively much hydrogen, hydrogen distribution in the strong field drift layer may change with the lapse of time (For example, hydrogen atoms drop out from the surface of the strong field drift layer.). Therefore, it is feared that the stability of the electron emitting efficiency as to time passing is deteriorated.
Thus, in the field emission-type electron source disclosed in the Japanese Patent Application of No. 10-272340, No. 10-272342 or No. 10-271876, although its cost can be lowered and electrons can be stably emitted with high efficiency in comparison with the field emission-type electron source disclosed in the Japanese Laid-Open Patent Publication No. 8-250766, it is expected to improve the electron emitting efficiency further more.